The present invention relates to methods and compositions for improving particulate flow. More particularly, the present invention relates to the use of degradable particulates as friction reducers that may reduce the potential stresses caused by increased particulate loading in fluids.
Fluids comprising solid particulates often are used in a variety of applications performed in subterranean formations. Such applications include, but are not limited to, drilling operations, production stimulation operations (e.g., hydraulic fracturing) and well completion operations (e.g., gravel packing). Fluids containing solid particulates are also used in a variety of surface applications as well.
The term “particulate(s),” as used herein, refers to particles having a defined physical shape as well as those with irregular geometries, including any particles having the physical shape of platelets, shavings, fibers, flakes, ribbons, rods, strips, spheres, spheroids, toroids, pellets, tablets, or any other physical shape.
In a hydraulic-fracturing operation, a type of fluid, commonly referred to as a “fracturing fluid”, may be placed in a subterranean formation at or above a pressure sufficient to create or enhance at least one fracture in the formation. Enhancing a fracture includes enlarging a pre-existing fracture in the formation. In some instances, a hydraulic-fracturing operation may involve pumping a proppant-free, viscous fluid (commonly referred to as a “pad fluid”) into a subterranean formation faster than the fluid can escape into the formation so that the pressure in the formation rises and the formation breaks, creating or enhancing one fractures in the subterranean formation. At a desired time, for example, once the fracture is formed or enlarged, particulates (commonly referred to as “proppant”) are generally placed into the fracture to form a proppant pack that may prevent the fracture from closing when the hydraulic pressure is released and thereby potentially enhance the conductivity of the formation.
In a gravel-packing operation, particulates (commonly referred to as “gravel”) may be carried to a portion of a well bore penetrating a subterranean formation by a carrier fluid, inter alia, to reduce the migration of unconsolidated formation particulates (e.g. formation sand) into the well bore. The carrier fluid may be viscosified, inter alia, to enhance certain properties (e.g., particulate suspension). Once the gravel has been placed into a gravel pack in the well bore or in a portion of the subterranean formation, the viscosity of the carrier fluid may be reduced, whereupon it may be returned to the surface and recovered. As used herein, the term “gravel pack” refers to the placement of particulates in and/or neighboring a portion of a subterranean formation so as to provide at least some degree of sand control, such as by packing the annulus between the subterranean formation and a screen disposed in the subterranean formation with particulates of a specific size designed to prevent the passage of formation sand. Gravel packs often are used to stabilize the formation while causing minimal impairment to well productivity. While screenless gravel-packing operations are becoming increasingly common, traditional gravel-packing operations commonly involve placing a gravel-pack screen in the well bore neighboring a desired portion of the subterranean formation, and packing the surrounding annulus between the screen and the well bore with gravel particulates that are sized to prevent and inhibit the passage of formation solids through the gravel pack with produced fluids. The gravel-pack screen is generally a filter assembly used to support and retain the gravel particulates placed during the gravel-packing operation. A wide range of sizes and screen configurations are available to suit the characteristics of the well bore, the production fluid, and the portion of the subterranean formation.
In some situations, hydraulic-fracturing operations and gravel-packing operations may be combined into a single operation to stimulate production and to reduce the production of unconsolidated formation particulates. Such treatments are often referred to as “frac-pack” operations. In some cases, these treatments are completed with a gravel-pack screen assembly in place with the fracturing fluid being pumped through the annular space between the casing and screen. In such a situation, the fracturing operation may end in a screen-out condition creating an annular gravel pack between the screen and casing.
In these and other operations involving a particulate-laden fluid, an upper limit may exist as to the optimum amount of particulates that can be suspended and successfully carried in the fluid. The flow of dispersions of particulates in a liquid may become increasingly difficult as the volume fraction of particulates increases, e.g., both the steady shear viscosity and the residual stress within the dispersion may increase as the volume fraction of particles increases. The increase in steady shear viscosity and/or residual stress generally is not linear; rather, it generally increases as the solids content approaches maximum packing (for fluids having a particle size distribution that is monodisperse, maximum packing of solids is known to be about 66% by volume of the dispersion). During the flow of concentrated dispersions of solids through a container or channel (e.g., a laboratory test tube or a subterranean fracture), the solid particles may form bridges across the inner diameter of the container or channel, thereby blocking or impairing the flow. This tendency to form bridges may increase as residual stress within the dispersion increases.
When this phenomenon occurs during a conventional subterranean application, e.g., a fracturing operation, this undesirable bridging of proppant particulates across the width of a fracture in a formation may tend to prematurely halt the deposit of the proppant particulates within the fracture. This bridging may block further flow of fracturing fluid into the fracture (thereby preventing continued propagation of the fracture). In other cases, the fracturing fluid may succeed in flowing around the blockage, and may continue (without the proppant particulates) to penetrate into the formation, thereby continuing to propagate the fracture for a time. In this latter case, however, the portion of the fracture that extends beyond the bridged proppant particulates generally will lack proppant particulates, and likely will undesirably re-close shortly after the termination of the fracturing operation, because it may lack the support necessary to maintain its integrity.
The addition of small silica particles (e.g., from nanometer to micron size) to particulate-laden fluids has been used to help alleviate stresses caused by increased particulate loading. For instance, the addition of small silica particles may allow increased particulate loading in a fluid, for example, up to or greater than about 55% solids by volume. While these small silica particles generally allow increased particulate loading, their use may have some drawbacks. For instance, after introduction into a well bore, these small silica particles may lodge themselves in formation pores, preslotted liners, screens, proppant packs, and/or gravel packs, preventing or reducing fluid flow there through. This may result in an undesirable reduction in well productivity, particularly in low permeability formations.